1. State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China
2. School of Ocean and Earth Sciences, Tongji University, Shanghai 200092, China
3. Laboratory of Gem and Technological Materials, Tongji University, Shanghai 200070, China
Abstract：Chemical composition and vibrational spectral characteristics of ensignia actinolite jade from Guangxi, China were investigated using electron microprobe(EPMA), Fourier transform infrared spectrometer(FT-IR) and laser Raman spectrometer(Raman). The site occupancy situation of metal cations with small ionic radii in the crystal structure of actinolite was discussed as well. The result indicates that ensignia actinolite jade from Guangxi is primarily composed of Fe-rich actinolite and minor ferroactinolite, and the high content of FeOT(Wt%：18.596%~26.791%)is the main reason for its high refractive index(1.64), density(3.12 g·cm-3) as well as black color in natural light and green color in transmission light. Due to the influence of Mg2+—Fe2+ isomorphous substitution in the crystal structure, the infrared absorption bands in 1 400~400 cm-1 and Raman peaks in 100~1 200 cm-1 all shift towards the low-frequency region with various frequency differences. In addition, four vibration bands(MgMgMg)OH(band A), (MgMgFe2+)OH(band B), (MgFe2+Fe2+)OH(band C), and (Fe2+Fe2+Fe2+)OH(band D)are displayed in mid- and near-infrared spectra as well as Raman spectra, among which the bands related to M—OH fundamental frequency vibrations appear at nearly 3 674, 3 660, 3 644 and 3 625 cm-1, respectively. While the bands related to M—OH multiple frequency vibrations appear at nearly 7 183, 7 154, 7 118 and 7 077 cm-1, respectively, and their relative intensity is A＜D＜B＜C. In addition, calculation based on chemical composition and integral area of M—OH infrared vibration bands reveals that Mg2+ and Fe2+ occupy firstly M2 and M1 sites respectively in the crystal lattice of ensignia actinolite jade from Guangxi. The distribution state of Mg2+ and Fe2+ in M1 and M3 sites is relatively orderly and the site-occupancy factors are close to 1. Comprehensive analysis demonstrates that the metamorphic grade of ensignia actinolite jade from Guangxi should belong to the greenschist facies category.
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